An internal combustion engine that is capable of switching an air-fuel ratio in operation is known. For example, a Patent Literature 1 discloses an internal combustion engine that is capable of switching the air fuel ratio between a stoichiometric air fuel ratio (namely, a theoretical air fuel ratio) and a lean air fuel ratio. Here, the internal combustion engine that is disclosed in the Patent Literature 1 operates with the stoichiometric air fuel ratio until an air intake quantity of the internal combustion engine reaches a predetermined value in order to prevent a deterioration of an exhaust property, when the air fuel ratio is switched from the stoichiometric air fuel ratio to the lean air fuel ratio.
Incidentally, there are a Patent Literature 2 and a Patent Literature 3 as background art documents that relate to the present invention. The Patent Literature 2 discloses a technology that delays an ignition timing while keeping the stoichiometric air fuel ratio during a period at which a control of switching a combustion mode is performed and that advances the ignition timing and switches the air fuel ratio to the lean air fuel ratio after the control of switching the combustion mode, when the combustion mode is switched from a non-supercharged stoichiometric combustion mode to a supercharged lean combustion mode. The Patent Literature 3 discloses a technology that increases the air intake quantity of the internal combustion engine and continuously changes the air fuel ratio to follow the change of the air intake quantity, when a switching from an operation with the stoichiometric air fuel ratio to an operation with a lean-side air fuel ratio (namely, the lean air fuel ratio).